Process for the production of a double-layer metal sheet and a more particularly stove enamelled shaped constructional member shaped therefrom, and also a deep-drawable double-layer metal sheet

ABSTRACT

The invention relates to a process for the production of a double-layer metal sheet, more particularly a lacquered shaped construction member comprising such a double layer metal sheet, for example, for the outer skin of a motor vehicle, having an intermediate layer ( 6 ) of epoxy resin glued to the two smooth cover sheets ( 7, 10 ). After the introduction between the cover sheets ( 7, 10 ) of the intermediate layer ( 6 ) and of spots ( 16 ), distributed over the surface, of an epoxy resin polymerizing at a lower temperature than the epoxy resin of the intermediate layer ( 6 ), the cover sheets ( 7, 10 ) are fixed spatially in relation to one another by the polymerization of the epoxy resin of the spots ( 16 ). Then the resulting composite is shaped. Finally the shaped constructional member ( 15 ) obtained is lacquered and the epoxy resin of the intermediate layer ( 6 ) polymerized during stove enamelling.

[0001] The invention relates to a process for the production of a double-layer metal sheet having an intermediate layer of an epoxy resin in the form of a hot melt glued to its cover sheets.

[0002] From such double-layer metal sheets shaped structural members, for example, are shaped which can be used in the bodywork of motor vehicles. For this reason they must have as low a weight as possible per unit of surface, accompanied by high buckling strength. The double-layer metal sheet must also be satisfactorily deformable, more particularly satisfactorily deep-drawable. Lastly, the shaped constructional member must not cause problems during lacquering, more particularly cathodic immersion lacquering, followed by the stoving of the lacquer.

[0003] The problem is known from U.S. Pat. No. 4,594,292 A1 that in a double-layer metal sheet having an intermediate layer of a resin which is glued to the cover sheets, the glued connection between the intermediate layer and the cover sheets is cancelled out by detachment under heavy deformations. To prevent this undesirable event while at the same time ensuring that during deformation the intermediate layer is not forced so strongly against the cover sheets that their outer zones become cracked, the intermediate layer consists of three different resin layers each having different properties. The core layer behaves plastically, while the outer layers consist of a ductile resin which does not flow under the deforming loadings occurring and is therefore not forced against the cover sheets of the double-layer metal sheet. In this way it should be possible to reach extreme deformations with bendings by up to 180° C. without the intermediate layer becoming loosened from the cover sheets and/or cracking taking place in the cover sheets.

[0004] However, shaped constructional members produced from such a double-layer metal sheet are unsuitable for stove enamelling, since the outer layers of the intermediate layer consist of a mixture of polyethylene and polypropylene i.e., thermoplastics—which become liquid at temperatures above 160° C. During stove enamelling, which is typically performed at temperatures of 180° C. and above for a duration of at least 30 minutes, such an intermediate layer would partly run out and the glued connection between the cover sheets and the intermediate layer, important for the buckling strength of the shaped constructional member, would be lost.

[0005] A double-layer metal sheet is also known (EP 0 598 428 A1) in which an intermediate layer of polypropylene is glued by an adhesive to the cover sheets. Such a double-layer metal sheet is supposed to be suitable for cold working to give a shaped structural member. The shaped constructional member is supposed to have a high dimensional stability after a heat treatment at at least 135° C. Since polypropylene is a thermoplastics which becomes liquid at temperatures above 160° C., the same thing applies to such a constructional member as to the previously-described constructional member, namely that with a treatment time of 30 minutes at a temperature of at least 180° C., as is customary in stove enamelling, the glued connection between the cover sheets and the intermediate layer is lost.

[0006] In an earlier German Patent Application (198 46 533.5), not previously published, a process is disclosed for the production of a stove enamelled shaped constructional member from a double-layer metal sheet, wherein the two cover sheets are glued to one another via an intermediate layer of an epoxy resin in the form of a hot melt. In that process the two cover sheets are fixed spatially in relation to one another after the introduction of the epoxy resin, without the epoxy resin used polymerising. The spatial fixing of the two cover sheets can be further improved by the feature that for one cover sheet use is made of a knobbed sheet which is welded to the other cover sheet by the tips of its knobs. The cover sheets thus spatially fixed in relation to one another, together with the intermediate layer, are then deformed to give the shaped constructional member. Finally, after the immersion lacquering of the deformed structural member, stove enamelling is performed at a temperature at which the epoxy resin of the intermediate layer also polymerises and therefore cures. It is true that in such a process the cover sheets are fixed in the optimum manner if they are welded to one another via the knobs, but such a double-layer sheet has the disadvantage that due to the welding points shown the flat cover sheet is not smooth. Such a double layer metal sheet is therefore unsuitable for the outer skin of a motor vehicle.

[0007] It is an object of the invention to provide a process for the production of a double-layer metal sheet, more particularly for stove-enamelled shaped constructional members, the two cover sheets of which are smooth, so that it can also be used as the outer skin of a motor vehicle. This problem is solved according to the invention by the features that after the introduction between the cover sheets of the intermediate layer of an epoxy resin polymerising (cross-linking) only at a higher temperature, more particularly stove enamelling temperatures, and of spots, distributed over the surface, of another epoxy resin which polymerises at a lower temperature than the polymerisation temperature of the epoxy resin of the intermediate layer and which can still be shaped to a limited extent in the polymerised state, the cover sheets are spatially fixed in relation to one another at spots in contact therewith by the heating of their polymerising epoxy resin to the lower temperature.

[0008] Preferable the double-layer metal sheet is further processed to give the more particularly stove-enamelled shaped constructional member by the following steps:

[0009] a) the double-layer metal sheet with the cover sheets attached to one another is deformed to give the shaped constructional member with the epoxy resin of the intermediate layer not yet polymerised,

[0010] and

[0011] b) after the deforming, more particularly after the lacquering of the deformed constructional member (15), said constructional member (15) is heated to the higher temperature, more particularly the stove enamelling temperature, at which the polymerisation of the epoxy resin of the remaining intermediate layer (6, 26, 27) takes place, more particularly simultaneously with the enamelling of the lacquer.

[0012] In the process according to the invention the selection of particular epoxy resins ensures on the one hand that the two cover sheets are satisfactorily fixed spatially in relation to one another, and on the other hand that in the deformation of the double-layer metal sheet to give the shaped constructional member, more particularly by deep drawing, due to its not yet crosslinked epoxy resin the large-area intermediate layer shares in the necessary deformations, as do the spots, due to the their almost negligible area in comparison with the intermediate layer and their still adequate deformability, even in the polymerised state, and the cover sheets are not impermissibly heavily loaded and/or the cover sheets do not become detached from the spots. Due to the small proportion of area taken up by the spots and the adjusted modulus of elasticity perpendicularly to the plane of the sheet (compressive loading), which in the polymerised state should be substantially equally to the modulus of elasticity of the not yet polymerised intermediate layer in the same direction, in contrast with knobs the spots are reliably not pressed through during deformation. The cover sheets therefore remain smooth, something which is absolutely necessary for the outer skin of a motor vehicle. At the same time, however, this also ensures that due to its tough state during deformation, the epoxy resin of the intermediate layer is not forced out between the cover sheets. It also makes sure that at the customary high temperatures which are operative for a prolonged period during stove enamelling, the epoxy resin melts without running out and polymerises, thus ensuring for the first time the final, whole-area firm gluing of the cover sheets to the intermediate layer and giving the shaped constructional member its optimum buckling strength after the epoxy resin has cooled.

[0013] Of course the invention is not limited to the epoxy resins described, but other materials with a similar behaviour are also suitable for the invention. It is important that during the deformation process such materials should fix the as a rule galvanised cover sheets without tearing off, something which presumes a certain flexibility. They should have a curing temperature of approximately 25° C. to 150° C. and a curing time which is shorter than the time of passage through a furnace. Lastly, in the cured state the spots should have substantially the same compressive strength as the intermediate layer in the still uncured state. Substances which are suitable for the spots include, for example, flexibilised epoxy resin adhesives, polyurethane adhesives and rubber adhesives.

[0014] The intermediate layer and the spots can be introduced in different manners.

[0015] In a first alternative the intermediate layer is provided with suitable receiving spaces prior to the introduction of the spots. This can be done by using as the intermediate layer a perforate foil which is not yet polymerised.

[0016] Different epoxy resins can be used for the spots. In the case of an epoxy resin which does not foam under the action of heat, the receiving spaces are filled. In contrast, with an epoxy resin which foams by heating they are only partially filled. In any case, it can be ensured that during the joining of the cover sheets and the intermediate layer, the epoxy resin of the spots contacts both cover sheets, so that they are spatially fixed as required after heating.

[0017] In a second alternative an intermediate layer is introduced over its whole area between the cover sheets, preferably by the coil coating process. The spots, which take the form of to some extent compression-resistant bodies, are scattered in. Contact between the spots and the cover sheets is effected more particularly by the feature that during the introduction of the intermediate layer and the spots between the cover sheets, followed by joining, due to their higher compressive strength the spots are forced into contact with the cover sheets by the intermediate layer.

[0018] The cover sheets can be fixed over the spots by the polymerisation of their epoxy resin in different manners. Inductive heating is suitable both in a continuous heating furnace and in steady conditions. A heating press which contacts the cover sheets is also suitable for steady heating. Continuous heating can be performed by means of heated rollers.

[0019] The invention also relates to a deep-drawable double-layer metal sheet having two metal cover sheets and an intermediate layer of epoxy resin disposed between the cover sheets, more particularly for use in a process of the kind disclosed. In such a double-layer metal sheet embedded in the intermediate layer, distributed over the cover sheets are adhesive spots which fix the sheets spatially in relation to one another and which consist of a polymerised epoxy resin whose polymerisation temperature is lower than the polymerisation temperature of the epoxy resin of the intermediate layer.

[0020] In such a double-layer metal sheet the cover sheet, which is subjected to heavier loadings during subsequent use, can be stronger, more particularly thicker in dimensions than the other cover sheet. As a result, a constructional member of optimum weight can be produced.

[0021] The invention will now be explained in greater detail with reference to the drawings, which show:

[0022]FIG. 1 the production process of a stove enamelled shaped constructional member from the phase of producing a double-layer metal sheet to the phase of stove enamelling.

[0023]FIG. 2 phases V to VII of the production process shown in FIG. 1, and

[0024]FIG. 3 a process for the introduction of an intermediate layer and spots between ribbons as cover sheets in a manner different from phases I to VII of the production process shown in FIG. 1.

[0025] In the production process shown in FIG. 1 a foil 2 of a not yet polymerised epoxy resin is produced for an intermediate layer by an extruder 1 (phase I). The foil 2 passes through a perforating apparatus 3 which produces in the foil 2, distributed over its area, receiving spaces 4 for spots of a different epoxy resin from that of the foil 2 intended for the intermediate layer (phase II). After the perforating apparatus 3 the foil 2 passes through shears 5, which cut the foil 2 to the length format of intermediate layers 6 (phase III).

[0026] The intermediate layer 6 is laid on a flat cover sheet 7 supplied from a stack of sheets 8 (phase IV). The receiving spaces 4 are then filled from a metering apparatus 9 with epoxy resin for the spots (phase V in FIG. 1 and bottom of FIG. 2). While the epoxy resin of the foil 2 polymerises at stove enamelling temperatures of normally above 160° C., the polymerisation temperature for the epoxy resin of the spots is lower, but above 70° C., at which, however, the epoxy resin of the foil 2 already becomes adhesive. In the embodiment shown in FIG. 2 the receiving spaces 4 are partially filled with a preferably thixotropically adjusted epoxy resin which foams under heat.

[0027] After the receiving spaces 4 have been filled, a flat cover sheet 10 is laid on which is supplied from a stack of sheets 11 (phase VI in FIG. 1 and central part of FIG. 2). The resulting composite comprising the two cover sheets 7, 10 and the intermediate layer 6 disposed therebetween, together with the epoxy resin present in the receiving spaces 4, then passes to a heating press 12 (phase VII in FIG. 1 and top part of FIG. 2). With the simultaneous application of heat and pressure the epoxy resin polymerises in the receiving spaces 4, possibly accompanied by foaming, so that the epoxy resin contacts the two cover sheets 7, 10 and sticks thereto, while although the epoxy resin of the intermediate layer 6 also sticks to the cover sheets, it does not polymerise. After cooling and curing the epoxy resin in the receiving spaces 4 forms the spots fixing the two cover sheets 7, 10 spatially in relation to one another.

[0028] The double-layer metal sheet 13 produced in this manner is then supplied to a deep-drawing installation 14 and shaped into the required constructional member 15 (phase VIII). During deep-drawing the still adequately deformable spots 16, consisting of polymerised epoxy resin distributed over the surface and occupying only a very small proportion thereof make themselves noticeable in as much as they fix the cover sheets 7, 10 spatially in relation to one another, but do not result in pressure marks in the cover sheets at the connecting places. The constructional member 15 thus shaped can therefore also be used as the outer skin of a motor vehicle.

[0029] The constructional member thus shaped then passes to a lacquering installation 17, more particularly a KTL immersion bath (phase IX). Then it is supplied to a furnace 18 in which the lacquer is stoved, the hitherto unpolymerised epoxy resin of the intermediate layer 6 being simultaneously polymerised (phase X). The result is a lacquered shaped constructional member which is suitable for the outer skin of a motor vehicle and which has low weight and high buckling strength.

[0030] The embodiment shown in FIG. 3 differs from that in FIG. 1 substantially by the way in which the intermediate layer and the spots are introduced between the cover sheets. To this end two cover sheets 20, 21 in ribbon shape are guided over two heated rollers 22, 23. Layers 26, 27 of epoxy resin are applied by applicators 24, 25 substantially at the apex of the rollers 22, 23 for the intermediate layer. The thickness of the layers 26, 27 is adjusted by squeegees 28, 29. Bodies 31 of epoxy resin are scattered, distributed over the width of the cover sheets 20, 21 by means of a pouring channel 30, into the gusset of the epoxy resin-coated, band-shaped cover sheets 20, 21. The bodies 31 have greater compressive strength than the layers 26, 27 and a thickness which is at least equal to the distance between the inner sides of the cover sheets 20, 21 in the nip of the rollers 22, 23. This ensures that the bodies 31 press through the layers 26, 27 of the softer epoxy resin and contact the cover sheets 20, 21. With adequate heating the epoxy resin of the bodies 31 polymerises and sticks to the inner sides of the cover sheets 20, 21, while the epoxy resin of the layers 26, 27 forming the intermediate layer does not polymerise, but the layers 26, 27 are bonded to one another. After cooling, the spots 32 formed by the bodies 31 and distributed over the area fix the cover sheets 20, 21 spatially in relation to one another. If instead of non-foaming epoxy resin, use is made of foaming epoxy resin for the formation of the spots 32, the diameter of the bodies 31 is of less importance. In that case the decisive factor is that the increased volume caused by foaming is large enough to contact the cover sheets 20, 21.

[0031] The double-layer metal sheet produced in this manner can then can be cut to length and further processed to give the stove enamelled shaped constructional members, in accordance with phases VIII to X in FIG. 1. 

1. A process for the production of a double layer metal sheet having an intermediate layer (6, 26, 27) of an epoxy resin in the form of a hot melt glued to its cover sheets (7, 10, 20, 21),. characterised in that after the introduction between the cover sheets (7, 10, 20, 21) of the intermediate layer (6, 26, 27) of an epoxy resin polymerising (cross-linking) only at a higher temperature, more particularly stove enamelling temperatures, and of spots (16, 32), distributed over the surface, of another epoxy resin which polymerises at a lower temperature than the polymerisation temperature of the epoxy resin of the intermediate layer (6, 26, 27) and which can still be shaped to a limited extent in the polymerised state, the cover sheets (7, 10, 20, 21) are spatially fixed in relation to one another at spots (16, 32) in contact therewith by the heating of their polymerising epoxy resin to the lower temperature.
 2. A process according to claim 1 and further processing to give a more particularly stove enamelled shaped constructional member, characterized by the following further steps: a) the double-layer metal sheet with the cover sheets (7, 10, 20, 21) attached to one another is deformed to give the shaped constructional member (15) with the epoxy resin of the intermediate layer (6, 26, 27) not yet polymerised, and b) after the deforming, more particularly after the lacquering of the deformed constructional member (15), said constructional member (15) is heated to the higher temperature, more particularly the stove enamelling temperature, at which the polymerisation of the epoxy resin of the remaining intermediate layer (6, 26, 27) takes place, more particularly simultaneously with the enamelling of the lacquer.
 3. A process according to claims 1 or 2, characterised in that the intermediate layer (6, 26, 27) is introduced over its whole area between the cover sheets (20, 21), more particularly by the coil coating process, small compression-resistant bodies (31) of epoxy resin being scattered in for the spots (32).
 4. A process according to claims 1 or 2, characterised in that the intermediate layer (7) is provided with suitable receiving spaces (4) prior to the introduction of the spots (16).
 5. A process according to claims 1 or 2, characterised in that the intermediate layer used is a perforate foil.
 6. A process according to claims 4 or 5, characterised in that the receiving spaces (4) are filled for the spots (16) with an epoxy resin which does not foam under the action of heat.
 7. A process according to claims 4 or 5, characterised in that the receiving spaces (4) are only partially filled for the spots (16) with an epoxy resin which foams under the action of heat.
 8. A process according to claim 1 , characterised in that during the introduction of the intermediate layer (26, 27) and the spots (32) between the cover sheets (20, 21), followed by joining, due to their higher compressive strength the spots (32) are forced into contact with the cover sheets (20, 21) by the intermediate layer (26, 27).
 9. A process according to one of claims 1 to 8 , characterised in that the double-layer metal sheet is heated to the lower temperature inductively or steadily by a heating press (12) which contacts the cover sheets (7, 10) or by heated rollers continuously contacting the cover sheets (20, 21).
 10. A deep-drawable double-layer metal sheet having two metal cover sheets (7, 10, 20, 21) and an intermediate layer (6, 26, 27) of epoxy resin disposed between the coversheets (7, 10, 20, 21), more particularly for use in a process according to claim 2 and more particularly in conjunction with one of claims 3 to 9 , characterised in that embedded in the intermediate layer (6, 22, 23), distributed over the cover sheets (7, 10, 20, 21) are adhesive spots (16, 32) which fix the sheets spatially in relation to one another and which consist of a polymerised epoxy resin whose polymerisation temperature is lower than the polymerisation temperature of the epoxy resin of the intermediate layer (6, 26, 27).
 11. A deep-drawable double-layer metal sheet according to claim 10 , characterised in that the two cover sheets (7, 10, 20, 21) have different strengths of material, more particularly different thicknesses. 